Taxonomy and morphology
Bacteria of the genus Enterococcus belong to the family Enterococcaceae and comprise 28 species. Historically, they were classified as group D streptococci due to the presence of a common group polysaccharide antigen. Two species are of the greatest clinical significance: E. faecalis, which accounts for the vast majority of infections, and E. faecium.
Morphologically, they are spherical or oval Gram-positive bacteria. In smears, they appear in pairs or short chains. Enterococci do not form spores or capsules, although some species possess flagella and are motile.
Cultural properties and differentiation from streptococci
Enterococci are chemoorganotrophs and facultative anaerobes. They are extremely undemanding regarding environmental conditions: they can multiply at temperatures ranging from 10 to 45 °C (optimum 37 °C), grow on simple nutrient media, and withstand alkaline pH shifts up to 9.6.
On blood agar, they form small (about 1 mm) grayish colonies, with hemolysis typically absent or manifesting as zones of incomplete clearing. When inoculated into milk with methylene blue, the bacteria decolorize the medium.
Main differential features distinguishing them from streptococci include their exceptional resistance to aggressive chemical factors. Enterococci readily grow in the presence of 40% bile and a high salt concentration (6.5% NaCl). Like streptococci, they are catalase-negative, but they actively ferment many carbohydrates with acid production.
Pathogenesis and clinical presentation
Enterococci lack pronounced organotropism, allowing them to affect various tissues. Pathogenicity factors (well-studied in E. faecalis) include adhesins, invasion enzymes (protease, gelatinase, promoting epithelial and heart valve colonization), and the cytolysin toxin.
Disease occurs when host resistance decreases, leading to bacterial translocation from natural biotopes into atypical cavities and tissues. This results in the development of purulent-inflammatory processes. The main clinical forms of opportunistic infections include:
- Urinary tract infections: most commonly associated with urological manipulations and catheterization.
- Bacteremia: an elevated risk is observed in patients undergoing hemodialysis.
- Nosocomial infections: frequently develop as complications in postoperative patients.
- Endocarditis: predominantly affects elderly individuals, patients with prosthetic heart valves, and organ transplant recipients.
Microbiological diagnostics and treatment
Research specimens include blood, urine, and wound discharge (depending on the localization of the process). The primary diagnostic method is bacteriological. The specimen is inoculated onto blood and selective nutrient media, after which a pure culture is isolated and identified based on its biochemical and cultural properties. Molecular genetic methods such as PCR and ribotyping are also used.
Due to the high intrinsic resistance of enterococci to most antibiotics (especially cephalosporins), antibiotic susceptibility testing is strictly mandatory in diagnostics.
Therapy is prescribed exclusively based on antibiotic susceptibility test results. Combinations of penicillins with $\beta$-lactamase inhibitors are often the drugs of choice (especially when $\beta$-lactamase-producing strains are identified). There is no specific prophylaxis; general sanitary and epidemiological measures are applied, similar to those for staphylococcal and streptococcal infections.
To easily remember how to differentiate enterococci from streptococci, use the "BS" rule (Bile & Salt): enterococci survive where others perish—in 40% Bile and 6.5% Salt.
Class and exam questions
Why were enterococci historically classified as streptococci?
They were classified as Group D streptococci because their cell wall contains a group polysaccharide antigen shared with them.
What are the main sources and transmission routes of enterococcal infection?
The source of infection is a human or animal. Transmission is characterized by multiple routes and mechanisms, and the bacteria lack strict tropism for specific organs.
Why is empiric treatment for enterococci often ineffective?
Enterococci possess high intrinsic resistance (e.g., to cephalosporins). Additionally, they frequently acquire resistance, as seen in VRE strains (vancomycin-resistant), which is why treatment must be selected strictly based on the antibiogram.
What else to study on this topic
- Mechanism of translocation of opportunistic microorganisms from GI tract biotopes
- Genetic mechanisms of vancomycin resistance (VRE) development
- The role of adhesins, gelatinase, and cytolysin in the pathogenesis of endocarditis
- Application of molecular genetic methods (PCR and ribotyping) in the identification of enterococci
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